1. Field of the Invention
This invention relates to a control apparatus for an internal combustion engine, and more particularly, to a control apparatus capable of variably controlling the air-fuel ratio of an air-fuel mixture supplied to an engine.
2. Description of the Related Art
Recently, gasoline engines have become commercially practical, which can be driven with a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio to improve the fuel efficiency.
In the lean air-fuel ratio operation for which a small amount of fuel relative to an intake air amount is set, a problem may occur such that ignitability for fuel around a spark plug is worsened, as compared to a case where the engine is driven with the stoichiometric air-fuel ratio. Thus, efforts have been made to improve the ignitability by changing configurations of the combustion chamber, intake port and the like or by changing a fuel injection system, so that an air-fuel mixture having a dense fuel concentration is locally collected in the vicinity of the spark plug.
In this case, the air-fuel ratio throughout a cylinder can be made lean and the air-fuel ratio can be controlled freely in a wide range.
To control the air-fuel ratio freely, however, the fuel injection amount, ignition timing and the like must be controlled smoothly and accurately depending on an engine operating state and its change. Further, fuel injection control and ignition timing control must be carried out, so as to suppress a change in the engine operating state caused by an increase or decrease in air-fuel ratio.
In this respect, a target air-fuel ratio and target ignition timing are determined in a manner accurately and immediately responding to the throttle opening degree, engine rotation speed, and their changes, in accordance with a throttle opening degree .theta.TH detected by a throttle valve opening sensor (TPS) and engine rotation speed Ne detected by an engine speed sensor, for instance.
Usually, the throttle valve opening sensor is mounted to the intake pipe of an engine and is hence caused to vibrate during the engine operation. Further, the throttle valve opening sensor has a movable portion which operates in response to the opening and closing actions of the throttle valve. As a result, the movable portion may be worn with use. If such a deterioration becomes noticeable with passage of time, the throttle valve opening sensor may be faulty.
If the throttle valve opening sensor, more generally, parameter detecting means for detecting an operating state of an accelerator member (engine speed adjusting member) such as an accelerator pedal, becomes faulty and operates abnormally, then the target air-fuel ratio and target ignition timing cannot be determined properly in accordance with an output signal of the sensor, so that the air-fuel ratio or fuel injection amount and the ignition timing cannot be controlled adequately. In such a case, the operating state of the engine becomes unstable so that drivability may be worsened.